1. Field of the Invention
The present invention relates to a plastic optical element used in an optical imaging system such as a laser digital copier, a laser printer, or a facsimile machine as well as an optical device such as a video camera. In particular, it relates to a plastic optical element made of a molded plastic article such as a plastic scan lens with a precise mirror face and a large, uneven thickness, and it relates to a nest structure for use in molding such plastic optical element, a die, an optical scan apparatus including the plastic optical element, and an image formation apparatus including the optical scan apparatus.
2. Description of Related Art
Heretofore, optical elements (lens, mirror) with laser beam imaging and correction functions have been commonly used in optical units of an optical imaging system such as a laser digital copier, a printer, a facsimile machine. In recent years, adoption of aspherical surfaces for the optical elements has improved their optical performance, and the optical elements have become moldable in complex shape by injection molding or injection compression molding. As a result, manufacture costs therefor have been in decrease. Generally, in order to precisely mold them in a desired shape by plastic molding including the above-mentioned injection molding or injection compression molding, it is preferable that in cooing solidification process of molten resin in a cavity of a die, pressure and temperature of the molten resin inside the cavity, that is, temperature distribution in the die has to be uniform.
When uneven resin temperature or local temperature distribution occurs in the die, or thermal contraction varies in the die, resulting in external defects such as sink marks in the molded plastic article. Increasing the amount of molten resin to fill in the cavity of the die by increasing injection pressure can be effective to solve the external defects. However, this may cause another problem of inner distortion of the molded plastic article, especially at a portion in thin thickness, adversely affecting the optical performance thereof.
Moreover, in molding an optical element in long length and uneven thickness, occurrence of sink marks will increase because the cooling speed, or thermal contraction of resin varies depending on a position of the optical element due to the unevenness in lens thickness, and temperature distribution need be uniform in the die in the longitudinal direction.
In molding such long, unevenly shaped optical element with the conventional nest structure which is composed of different members for an optical element body and a support portion connected with the optical element body, sink marks typically occur in a joint surface between the different members due to air flow into the joint surface when filling molten resin into the cavity of the die by injection. The air inflow locally decreases the temperature or adherence of the die, resulting in local thermal contraction, in other words, the sink marks in the molded plastic article as well as another external defect as air bubbles.
Japanese Patent No. 4108195 (Reference 1) discloses a high-precision molding method for molding long, unevenly shaped plastic optical element without remnant inner pressure of resin and inner distortion at as low manufacture cost as that for a thin molded plastic article by providing an incomplete transfer part in concave or convex shape on a surface other than a transfer surface.
Japanese Patent Nos. 3696420 (Reference 2) and 3512595 (Reference 3) disclose a concrete method for forming a concavity on a part of the surface other than the transfer surface of the plastic optical element. The method comprises the steps of preparing a pair of dies each with at least one or more cavities formed by cavity pieces and transfer surfaces, the cavity pieces forming surfaces including a non-transfer surface and having at least one or more vent holes and at least one or more communication ports in communication with the vent holes to supply compressed gas to an article and connected with a compressed gas supply device outside the die; heating the die at a temperature less than a softening temperature of resin and maintaining the temperature, filling molten resin heated at over the softening temperature in the cavities by injection; creating resin pressure on the transfer surface to get the resin in close contact with the transfer surface; supplying compressed gas to the resin in the cavities via the vent holes when the resin is cooled down below the softening temperature; and enforcedly defining voids between the cavity pieces with the vent holes and the resin to thereby form a non-transfer surface (also, disclosed in Japanese Laid-open Patent Application Publication No. 2002-337178).
However, there is a drawback in the above-mentioned method that for molding the plastic optical element using the conventional nest structure which is composed of different members for the optical element body and the support portion, the occurrence of sink marks increases compared with using a general molding method. The sink marks occur when compressed gas is supplied to the resin in the cavities due to the air flow into the joint surfaces between the members which causes local decrease in temperature or adherence of the die and results in local thermal contraction, or the sink marks. The air flow also causes the air bubbles.
When the plastic optical element is an fθ lens or the like used in the current optical scan apparatus, it is especially required to reduce the external optical defects while maintaining the optical performance of the lens body and all or part of the support portion, since a part of the support portion is integrally formed with the lens body having the transfer surface and used for having a laser beam pass therethrough for determining scan start timing.
In order to solve the above problem, Japanese Laid-open Patent Application Publication No. 2006-168285 discloses a technique to integrally form a nest structure for the optical element body and the support portion connected therewith. However, there still remains a problem in this technique that the optical element body cannot be easily released from the nest structure. In other words, it is not possible to continuously mold the plastic optical element in main scan direction which should be smoothly molded compared with sub scan direction. The plastic optical element cannot be molded in the same nest structure.